1. Field of the Invention
The present invention generally relates to a switching power converter and a current trigger circuit, and more particularly, to a switching power converter and a current trigger circuit capable of comparing a bias voltage with a voltage difference in a detecting element to detect a current flowing through the detecting element.
2. Description of Related Art
Please refer to FIG. 1, which is a schematic diagram of a buck DC/DC converting circuit for converting an input voltage VIN to an output voltage VOUT in a related art. The buck DC/DC converting circuit includes a switch SW, an inductor L, a capacitor C, a diode D, a detecting resistor Ri, and a controller CON. The switch SW is switched according to a control signal CONTROL generated by the controller CON to control an amount of electric power transmitted by the input voltage VIN. While the switch SW is conductive, an inductive current IL flows to the capacitor C through the inductor L and is stored in the capacitor C. While the switch SW is cut-off, electric power stored in the inductor L is transmitted to the capacitor C through the diode D and then stored in the capacitor C. The capacitor C provides power to a load (not shown) with the output voltage VOUT.
In order to avoid unnecessary power consumption due to the saturated inductor L or avoid elements of the buck DC/DC converting circuit from being damaged by an excess current, the controller CON detects an amount of the inductive current IL passing through the inductor L by means of the detecting resistor Ri. The controller CON has a comparator which compares a voltage difference VS across the detecting resistor Ri with a reference voltage Vref generated by a reference voltage generator. While the inductive current IL flows through the detecting resistor Ri to have the voltage difference VS across the detecting resistor Ri greater than the reference voltage Vref, the switch SW is cut-off. Owing to the noises on the circuit, voltage levels of the two terminals at the detecting resistor Ri have variations. As a result, the detection easily fails, and the erroneous operation is induced. Therefore, in practice, the voltage difference VS generally has to be greater than 0.2 voltage (even greater than 0.5 voltage or 0.8 voltage), so that the controller CON can effectively avoid the erroneous operation. With current trend that operation voltages of electric elements become lower and lower, the voltage difference across the detecting resistor has a larger percentage and so conversion efficiency of the buck DC/DC converting circuit becomes lower. How to provide a detecting circuit with high conversion efficiency thus becomes an inevitable issue in compliance with the current trend that the operation voltages of the electric elements become lower and lower.